Salt water (for example, saline) has indeed been extensively used by medical caregivers for wound management, particularly to clean out foreign materials from a wound or to cleanse the wound before dressing it. But this saline water that is medically used is sterile, which means it is free of any microorganisms or foreign materials that may infect the wound, on the other hand, the saltwater from the ocean is far from being sterile. They contain numerous pathogenic organisms and debris that might cause severe infections.
Although some people might not easily get infected, there are certain things to consider before you decide to head into the ocean with an open cut or wound:
It's important to remember not all seawater surfaces are the same. As you go deeper, the microbial ecosystem changes too. While certain parts of the ocean certainly encourage lots of bacteria, naturally or due to anthropogenic activities.
The water in estuaries and near the land, rocks or coral holds heavy amounts of bacteria as environmentally it is favorable with ample sun rays, oxygen and food which are present in the oceans when fisheries, mines, farms, stormwater and sewage plants are nearby drained. This becomes especially risky after a period of heavy rainfall.
Bacterial growth is also seen to be more in tropical waters as the warmer water houses more favorable conditions.
These can contain the infamous 'flesh-eating bugs', which are marine Vibrio. These organisms are halophilic i.e. they can sustain high salt concentrations and can lead to sepsis, gastroenteritis and soft tissue infections. People who have come into contact with marine vibrio, a type of bacteria that is responsible for causing cholera, which led to fatal infections through open sores and cuts.
Sea water contains several harmful bacteria and unwanted infections causing debris that will only worsen an open wound. Therefore, it is advisable to never consider cleaning the wound using seawater, rather if you're swimming in a water body and get a few minor cuts, it's a good idea to clean them with clean bottled water right at that moment and cover it with a clean cloth. Once you get back, clean the wound with sterile saline water or antiseptic, even if they appear fine. This is especially crucial if the cut happens on coming in contact with any rocks or coral while you're swimming, as these tend to harbor innumerable bacteria.